The use of a step-down converter, also referred to as a buck-converter, for the control of LEDs is known in principle. As shown in FIG. 1, a switch S1 is closed and opened in alternation, wherein, in its activated condition, a coil LBuck is energised. In turn, in the deactivated condition of the switch S1, the energy accumulated in the coil LBuck is discharged via the LED system.
The switch S1 is clocked by a control unit (not shown). This control unit monitors the current through the switch S1 during the activation phase of the switch S1 via a measuring resistor RSHUNT connected in series to the switch S1. As soon as the voltage which is picked up via the measuring resistor RSHUNT reaches a given maximal value, the switch S1 is opened.
Furthermore, an indirect detection of the voltage VLED across the LED system is provided. The voltage detection is implemented in the freewheeling phase of the switch S1, that is, when the switch S1 is open, that is, not conducting, wherein, in this phase, a current flows through the LED system, a diode D2 and the coil LBuck embodied as the primary side of a transformer T1.
FIG. 2 shows the characteristic of electrical parameters from the circuit according to FIG. 1. With closed switch S1, the following equation applies for the voltage V′LED across the secondary side L2 of the transformer T1:V′LED=(VIN−VLED)/r wherein VIN denotes the input voltage of the step-down converter, and r denotes the transformer ratio of the transformer T1.
During the freewheeling phase, the following equation once again applies approximately:V′LED=VLED/r 
According to the prior art, the coil LBuck is embodied as a primary winding of the transformer T1, wherein the secondary winding L2 of the transformer T1 serves for the indirect detection of the voltage VLED across the LED system. Accordingly, a secondary winding L2 is coupled to the primary winding LBuck, by means of which the LED voltage can be measured in the freewheeling phase of the switch S1, because the LED voltage is fully present across this primary winding LBuck in the freewheeling phase.
The secondary winding L2 is connected, on the one hand, to ground and, on the other hand, to a resistor RCHG. An envelope-curve demodulator comprising a diode D1, a capacitor C1 and a resistor RDISCHG are connected in series to this resistor RCHG. These three components form an envelope-curve demodulator for the voltage V′LED of the secondary winding L2. The diode D1 allows only one polarity of the high-frequency voltage V′LED to pass. The parallel configuration of the capacitor C1 and of the resistor RDISCHG forms a low-pass filter for the removal of the high-frequency signal. The corresponding characteristic of the voltage VADC present in this low-pass filter or respectively in the envelope-curve demodulator is shown in FIG. 2.
It is already known that this voltage VADC is supplied to the control unit in order to determine the activation time of the switch S1. More particularly, the voltage VADC reproduces the voltage VLED across the LED system during the freewheeling phase of the switch S1, wherein the transformer ratio r and the voltage VF across the diode D1 must then also be taken into consideration. This diode voltage VF depends upon the current through the diode D1. Since the current through the diode D1 declines almost to zero with a charged capacitor C1, the diode voltage VF is dependent upon the deactivation duration of the switch S1.
The transformer T1 and the capacitor C1 together form a resonant circuit which, in turn, can cause harmonics or respectively electromagnetic disturbances. Furthermore, the fact that the envelope-curve demodulator comprising the diode and the low-pass filter provides a temperature-dependent and operating-point-dependent voltage error is also problematic. As a result, considerable measurement errors occur, which cannot be corrected.